Astrochemistry at work in the L1157-B1 shock: acetaldehyde formation

TL;DR

This study presents high-resolution imaging of acetaldehyde in a protostellar shock, revealing its formation is likely driven by gas phase reactions involving iced hydrocarbons released from dust grains.

Contribution

First high-resolution images of acetaldehyde in a protostellar shock, linking its formation to gas phase reactions and dust grain chemistry.

Findings

Acetaldehyde emission closely follows the outflow cavity.

High acetaldehyde abundance similar to hot-corinos.

Gas phase formation requires significant iced hydrocarbons.

Abstract

The formation of complex organic molecules (COMs) in protostellar environments is a hotly debated topic. In particular, the relative importance of the gas phase processes as compared to a direct formation of COMs on the dust grain surfaces is so far unknown. We report here the first high-resolution images of acetaldehyde (CH$_3$CHO) emission towards the chemically rich protostellar shock L1157-B1, obtained at 2 mm with the IRAM Plateau de Bure interferometer. Six blueshifted CH$_3$CHO lines with $E_{\rm u}$ = 26-35 K have been detected. The acetaldehyde spatial distribution follows the young ($\sim$ 2000 yr) outflow cavity produced by the impact of the jet with the ambient medium, indicating that this COM is closely associated with the region enriched by iced species evaporated from dust mantles and released into the gas phase. A high CH$_3$CHO relative abundance, 2-3 $\times$ 10$^{-8}$, is inferred, similarly to what found in hot-corinos. Astrochemical modelling indicates that gas phase reactions can produce the observed quantity of acetaldehyde only if a large fraction of carbon, of the order of 0.1%, is locked into iced hydrocarbons.